Gemini CLI 63-character MCP tool names: do Sume tool names fit?
Gemini CLI builds names as mcp_server_tool and truncates past 63 characters. Sume's longest documented tool names fit if the server name stays short.
Yes, Sume's tool names fit. Gemini CLI turns every MCP tool into mcp_{serverName}_{toolName} and truncates names longer than 63 characters (read 2026-10-02). The longest hosted Sume tool name in the docs is 36 characters, so with the server named sume the result is 45, well under the limit.
The naming rules, as documented
Gemini's MCP page states the rules in one place. They apply to every server you add, not only Sume.
| Rule | What the docs say | Effect on Sume |
|---|---|---|
| Format | Every MCP tool gets the fully qualified name mcp_{serverName}_{toolName} | mcp_sume_generate_image |
| Underscores | Do not use underscores in server names | Name the server sume, not sume_prod |
| Characters | Anything other than letters, numbers, _, -, . and : becomes _ | Sume's hyphenated names keep their hyphens |
| Length | Names over 63 characters are truncated with middle replacement | Not reached with a short server name |
The arithmetic
The prefix mcp_ is 4 characters and the joining underscore is 1, so a name is 5 characters plus the server name plus the tool name. Among the tools listed in Sume's docs, avatar-video-previews_generate_video is 36 characters, and avatar-video-previews_create, video-caption-overlay_create and generation_admission_preview are 28 each (MCP tools and gates).
For the 36-character name, the budget leaves 63 - 5 - 36 = 22 characters for the server name. So a server called sume (4) is nowhere near it, and a server name up to 22 characters would still pass for every documented tool.
Why this matters in practice
Truncation with middle replacement means two long names could collapse toward each other, and a policy rule written for the full name would no longer match. Sume's tools share long prefixes, such as avatar-video-previews_... and timeline_..., so staying under the cap keeps each tool's full name intact and your approval rules predictable.
The practical advice is short. Use sume as the server name, avoid underscores in it, and write policy rules against mcp_sume_<tool>. If you rename the server for an environment, count again with the formula above.
Limits of this check
To confirm in your own session, run /mcp in Gemini CLI after connecting and compare the listed names with the formula (MCP quickstart).
- I only checked names printed in Sume's docs; call
tools_listfor the live set in your session, which may differ. - The 63-character rule comes from Gemini's docs, not from the MCP specification.
- This post is about names; it does not change what an API key or OAuth scope can call.
Sources
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